1. Field of the Invention
The present invention relates to a method of in-situ monitoring a reduction process of uranium oxides to uranium metal by use of lithium metal.
2. Description of the Related Art
Nuclear reactors, pressurized water reactors (PWR) and pressurized heavy-water reactors (PHWR), using UO2 as a nuclear fuel, generate electricity through a fission process and additionally produce fission products. Thus, spent fuel is composed of uranium oxides and fission products. However, since some elements of the fission products may absorb neutrons required to maintain the fission, oxide fuel performance is decreased in combustion efficiency. Eventually, the spent fuel with low efficiency is permanently disposed of to high-level radioactive wastes or re-treated to be recycled to uranium. Typically represented by PUREX (Plutonium Uranium Recovery by EXtraction), a wet re-treatment process is employed to purely separate plutonium, which is applicable exclusively for manufacturing nuclear weapons. Hence, to achieve nuclear proliferation resistance, the wet re-treatment process is not allowed to be performed, with the exception of United States and some countries. On the other hand, a molten salt based process is known not to enable plutonium to be purely separated, and has the advantage of proliferation resistance. Accordingly, the molten salt process, which serves to remove the fission products from uranium following the reduction of the spent oxide fuel to uranium metal, is under study so as to be used for recycling uranium as a fuel or disposing such uranium to intermediate- or low-level radioactive wastes.
Useful for recycling the uranium fuel, a reduction process of uranium oxide by use of lithium in the presence of a high-temperature molten salt is represented by Reaction 1, below:

As shown in Reaction 1, after lithium chloride to be used as the molten salt and the spent oxide fuel are supplied to a reduction reactor, the temperature in the reactor is increased to 650° C. to melt lithium chloride, which functions as a solvent in the molten state. Subsequently, lithium metal is added into the reactor under inert (Ar) atmosphere with stirring of the molten salt, whereby lithium metal reacts with uranium oxide, therefore resulting in reduction of uranium oxide to uranium metal and production of lithium oxide as a by-product. In this case, since uranium metal is not dissolved in the salt, it is settled down as a solid state. Also, lithium oxide which is the by-product is dissolved, with 8.7 wt % solubility in the salt at 650° C. Eventually, uranium metal powders can be obtained by removal of the molten salt composed of lithium chloride and parts of lithium oxide.
Meanwhile, since the molten salt based process for use in separating the fission products from uranium is developed in the absence of oxides, uranium cannot be efficiently recovered in the presence of larger amounts of oxides. Hence, the complete conversion of the spent oxide fuel to metal is regarded to be important. For this, while the conversion of the spent oxide fuel to metal is confirmed at any time, the completion of the reduction process is monitored.
In this regard, a conventional monitoring method for a reduction process of uranium oxide is exemplified by a wet analytical method, which includes collecting a lithium chloride sample molten at high temperatures during the reduction process, cooling and crushing the sample under inert gas atmosphere to weigh the sample, measuring the volume of hydrogen produced due to dissolve the sample with distilled water, and then titrating the solution with an acid.
Lithium metal has a 0.5 mol % solubility in a pure lithium chloride molten at 650° C. As the concentration of lithium oxide increases, solubility of lithium metal increases. Accordingly, when lithium oxide is dissolved in distilled water and then titrated with an acid, the amount of lithium hydroxide resulting from the reaction of lithium metal dissolved in lithium chloride with water should be corrected.
Specifically, according to the wet analytical method, the volume of hydrogen generated by the reaction of lithium metal contained in the lithium chloride sample with distilled water is measured by use of a hydrogen volumetric unit, to preferentially measure the amount of lithium metal contained in the sample. Thereafter, the solution produced in the hydrogen volumetric unit is titrated with an acid to obtain a total amount of lithium oxide, from which the amount of lithium metal obtained by the measurement of the hydrogen volume is subtracted, thereby accurately determining an amount of lithium oxide produced by uranium oxide.
However, the above wet analytical method is disadvantageous in that the amount of lithium metal is difficult to be accurately measured, since the temperature should be maintained at a predetermined level to measure the volume of hydrogen. Further, the above method has the drawbacks, such as impossibility of direct measurement of a conversion yield of uranium from uranium oxides upon the reduction process, and complicated analytical procedures of collecting the sample, weighing the cooled sample, crushing and dissolving the sample, etc.